SAN FRANCISCO — When the 156 players tee off in today’s U.S. Open first round at Olympic Club, 154 of them will be an afterthought.

The eyes of the golfing and sporting world — at least for the first two days of this 112th Open — will be fixated on Tiger Woods and Phil Mickelson, who will be grouped together for the opening two rounds.

Woods’ reaction to playing alongside Mickelson: indifference.

“I don’t think we’re going to talk about a lot,” Woods said. “This is a major championship. We’ve got work to do. Any extra motivation? No. I’m just trying to get out there and position myself for Sunday.”

“I get excited to play with Tiger,’’ Mickelson said. “I love it. He gets the best out of me.’’

Mickelson’s exuberance likely can be traced to his recent success while playing against Woods; he’s outdueled Woods in seven of the last 10 times they’ve been paired together.

The burn marks from Mickelson mowing down Woods en route to winning the AT&T Pebble Beach Pro-Am in February when they were last paired together are probably still evident on the back of Woods’ Sunday red shirt.

Mickelson, storming from behind for his only victory of the year, shot 64 to Woods’ 75 in the final round.

Why the turnaround for Mickelson, who struggled earlier in his career in the presence of Woods, going 5-10-3 against him in their first 18 meetings in the same group?

The “Common Denominator’’ has the answer.

Butch Harmon has coached both Woods and Mickelson. During Woods’ reign over Mickelson, Harmon was in Woods’ camp. Since 2010, which covers some of Mickelson’s success against Woods, Harmon has been coaching Mickelson.

“The big thing I had to teach him was to embrace it,’’ Harmon told The Post yesterday. “I told him, ‘You two, for the last 20 years, are the best American players the game has seen and when you get a chance to go head-to-head you’ve got to live for it. This is the guy that you respect the most in the game. Have fun with it.’

“And he really has. He really looks forward to it. He loves the challenge. It brings out the best in him where in the past, I’m not going to say anyone has ever intimidated Phil, but when Tiger was on his great run back in ’99, 2000, 2001, I think everybody was a little intimidated.

“I also helped him with some things in the past that Tiger used to do — not anything illegal or outside the rules, but little gamesmanship nuances he needed to be made aware of.’’

Harmon referred to Woods putting out when he could so that his playing partner would have to deal with the distraction of the large galleries scurrying to the next tee, playing fast when playing with a slow player and vice-versa.

“Phil said to me, ‘Did that actually happen?’ ’’ Harmon said. “I said, ‘Well, we’ve been doing it to you for five years. Yeah, it happened.’ ’’

“The one player I’m most concerned about if I play my best golf that may have a chance to beat me is Tiger,’’ Mickelson said. “When we first started playing together, I don’t know what it was exactly, but I didn’t play my best when we were paired together. The last five years or so I’ve been able to focus clearly when we play. I’ve played some of my better golf these last five years with him.

“The feeling and the excitement I got when I found out I was paired with Tiger the final round at Pebble got me excited and focused. I felt a similar feeling when I found out we were paired together here and I’m looking forward to the pairing.’’